The present invention relates to an apparatus for measuring a physical property, such as thickness of a sheet material. More particularly, the invention relates to a caliper gauge for measuring the thickness of a sheet of paper which is in the process of being manufactured by a papermaking machine and, therefore, is moving at a high rate of speed through the caliper gauge.
Various types of caliper gauges are known in sensor technology and are used for measuring the thickness of rapidly moving sheet material. One type of caliper gauge is called a "contacting caliper gauge." Contacting caliper gauges typically have two opposing pads which are forced into contact with opposite sides of a sheet. The distance between the pads is measured and directly relates to the sheet thickness or "caliper."
Under some situations, contacting caliper gauges may suffer from certain shortcomings. For example, U.S. paper manufacturers have formed a association (TAPPI) to promote uniform standards for the paper industry. Since paper is somewhat compressible, the TAPPI standard for measuring paper sheet caliper requires that the measurement be based upon results obtained from a contacting caliper gauge having opposing pads which exert a certain specified pressure on the paper sheet. Unfortunately, the requirement that the caliper gauge contact the sheet under pressure poses a problem when measuring the caliper of particularly lightweight, thin or weak paper. Under the required pressure, the sheet contacting pads have a tendency to tear such sheets. This is particularly true when a sheet imperfection causes an abrupt increase in thickness.
Certain prior caliper gauge pads are essentially disk-shaped, with smooth, rounded pad edges to avoid snagging imperfections in the sheet. These caliper pad designs may exhibit limitations in modern paper mills, where papermaking process speeds can approach 60 mph. The rapidly moving sheet drags with it a boundary layer of air, near the surface of the sheet, at approximately the same speed as the sheet. As the sheet passes between the opposing caliper pads, the boundary layer of air on both sides of the sheet creates a lifting effect, which tends to force the pads away from the sheet surface. Thus, at high paper speeds, the pads of a "contacting" caliper gauge may actually fail to contact the sheet. However, as previously mentioned, conventional caliper gauges determine sheet thickness based on the measured distance between opposing pads. Thus, the lifting effect can induce an erroneous caliper measurement by making the sheet appear thicker than it actually is. The lifting effect increases as the sheet speed, and hence the speed of the boundary layer air, increases.
The force on the contacting pads can be increased to overcome the lifting effect, but if the process speed is reduced significantly, the lifting effect of the boundary air provides less opposition to the force of the contacting pads. Thus, when process speed is reduced, the pressure of the contacting pads on the sheet increases, increasing the tendency of the pads to tear thin or lightweight papers.
Another caliper designer recognized that he must consider the aerodynamic design of the caliper pads if the pads were to be maintained on or near the sheet surface with relatively little external force. This previous caliper gauge having aerodynamically designed caliper pads is disclosed in the U.S. Pat. No. 4,901,445 to Matthew G. Boissevain et al., assigned to Measurex Corporation which is incorporated herein by reference.